Perceptual learning refers to changes in sensory abilities that occur through training and is thought to be an important process that helps us adapt to our ever-changing physical environment. A fundamental aspect of perceptual learning is that it is subserved by plasticity in sensory processing stages and is, by and large, a form of implicit learning. Thus, although there is a wide body of evidence showing that external reinforcement, such as response feedback, results in increased performance and learning rates during many types of learning (Goldstein & Rittenhouse,
1954; Goodman & Wood,
2004; Pavlov, Gantt, & Folbort,
1928; Schultz,
2000), the role of external reinforcement is still unclear in perceptual learning. For instance, while some studies indicate a clear role of external reinforcement in perceptual learning (Herzog & Fahle,
1997), many studies find that perceptual learning occurs robustly in the absence of external reinforcement (Ball & Sekuler,
1987; Fahle, Edelman, & Poggio,
1995; Karni & Sagi,
1991; McKee & Westheimer,
1978; Shiu & Pashler,
1992). Furthermore, recent research has shown that perceptual learning of optical motion can occur as a result of mere exposure to a subliminal stimulus, without external reinforcement, without the subject actively attending to the motion stimulus, or with the motion stimulus being a relevant feature of the particular task (Seitz & Watanabe,
2003; Watanabe, Nanez, & Sasaki,
2001). These studies raise the question concerning under what conditions feedback may be required to produce perceptual learning.